The invention relates to a holographic display in accordance with the generically determinative features of claim 1.
In a generic holographic display which is disclosed in DE 3735983 C2, for the purpose of recording a hologram image with the aid of which a hologram image can be obtained behind the hologram plate, the surface of a hologram plate being illuminated by a reference laser light of a spherical wave and the rear of the hologram plate being illuminated by an object laser light which is scattered by a diffuser and transilluminates an object, there is a superimposition of the light bundles on the hologram photographic dry plate such that interference fringes are recorded which extend in a direction of the thickness of the hologram emulsion layer. In order to reproduce the hologram image thus recorded, the hologram plate is illuminated by the light of an optical source in the same direction as the reference light which was used to record the hologram. Only the light of a wavelength which satisfies the Bragge condition is reflected and so a virtual image of the object is produced behind the holoplate. In order to generate volume holograms, use is made of a storage medium for holographic recordings which is exposed with an object beam and reference beam. Interference between object and reference beams inside the hologram volume produces a sequence of Bragg planes. The ability of the volume holograms to store a plurality of images simultaneously permits the production of coloured holograms. The recording of holograms requires lasers which can emit the three primary colours of blue, green and red as accurately as possible. The three beam clusters simultaneously expose the recording medium at equal angles. Three holograms are stored simultaneously in the volume hologram after the exposure. The reproduction of the coloured hologram is served by the distinguishing feature, characteristic of volume holograms, that each partial hologram is to be reconstructed solely by the colour with which it was recorded. The reproduction is therefore performed best with the lasers already used in the recording. The three reconstructed colour separations are superimposed to form the coloured, faithful image if the colour components are correctly weighted.
It is disadvantageous in this type of holographic display that the requirements made of the recording technology and the projection device become more exacting as the screen area increases. In the case of front projection, the soiling, possible at any time, of the screen leads to impairment of viewing. Particularly in the case of reproduction, it is impermissible for the projector to move relative to the screen. Consequently, it is necessary to guarantee a stable, vibrationally isolated suspension. Moreover, the beam path can be interrupted by persons and objects. Despite folding of the beam path, the structure requires a substantial installation depth.
The object of the invention is to provide a holographic display for generating virtual images which can have a large screen area without worsening the image quality, it being possible for the screen area to have any desired surface.
According to the invention, the object is achieved by means of the features of claim 1. Advantageous refinements and developments of the subject matter of the invention are characterized by the features of the subclaims.
A substantial advantage of these refinements resides in that the display can have a large screen area without causing the image quality to suffer. The virtual image plane can be set such that the visual image distance is variable. A stereo representation is possible without spectacles. Furthermore, the holographic display has a full-colour representation with a high contrast performance even in daylight, this being independent of the shape and geometry of the virtual screen. The display is insensitive to extraneous light, and so a high contrast is provided even in daylight. The holographic display is flexible and can be accommodated in the most varied types of installation space. Furthermore, a plurality of pictorial representations are obtained simultaneously in different directions of view.